My boat is listed at 340 on the main and 350 on the jib, which is a 150% roller, and then 490 on the screacher, which is a light air sail that PCI says is the equiv of 200% if it was a jib. Doing that math doesn't work out so good. because 280 is the I x J / 2 number and 280 x 2 is not 490
In fact, none of their numbers really works out.

The mast top is 40ft from the deck, and 14 ft from mast to furler. Basic formula Is I x J /2 , or 14x40 / 2 or 280 sq ft.
A 150% jib should then be 420, and a 200% should be 560.

It's all marketing hype

In fact Practical Sailor magazine (known for cutting through the hype) lists the Sail area of the Gemini as 540 sq ft - I have no idea how they came up with that as 340 + 280 = 620, unless the roached area of the main doesn't count either and it should because it's always in play.